Would have bet the house that would work. Because your trying to pass /quiet to the install, not to runnas.

Sometimes what I do to debug these long funky concatenated strings is to just print it out to the console, to see how its being passed, like this:

$= SetOption("WrapAtEol", "On")

? $runaspath + ' /user:' + @wksta + '\Administrator ' + '"$winpath"' + ' /pass:xxxxxxxx'

The WrapAtEol ensures that the console output doesn't wrap onto itself. Try it and paste your output for us to see (minus the password of course ;0))